1. Field of the Invention
The present invention relates to an image reading apparatus for photoelectrically reading an image of an original.
2. Related Background Art
Various image reading apparatuses have been proposed as an apparatus for inputting an image into a facsimile machine, digital copying machine, or a computer, wherein the image of an original is exposed by a light source and an image signal is formed by photoelectric conversion of the light reflected by the original.
In such image reading apparatus, one of the points requiring caution for accurate reading of the image with proper density and contrast is the adjustment of the light quantity of the light source used for exposure of the original at a proper level. The assignee of the present invention proposed systems for adjusting the light quantity of the light source at the proper level in U.S. Pat. No. 4,677,287 and U.S. Pat. No. 4,691,365.
However, for example, when a fluorescent lamp is used for the light source, there is a drawback in that a longer waiting time is required at low temperature (FIG. 1B) in comparison with such waiting time at normal temperature (FIG. 1A) until the light quantity reaches the proper level. Thus the throughput in unit length of time of reading decreases.
In order to shorten such waiting time, it may be useful to provide a heater for heating the light source, but the cost of the apparatus should increase too much.